[RE-wrenches] Unusual problem with SMA TL inverter on metal roof

[boB at midnitesolar.com]

Interesting app note.  Interesting problem.

How far from this metal roof (grounded I presume) is the bottom of the 
modules ?

This app note is mainly talking about the area of the module from the 
mounting bracket which should
be quite a bit less than the modules area from a distance above a 
grounded metal plate like this roof.

How far above the metal roof are the modules ?  If they are real close, 
raising them slightly might
be enough to lower that capacitance enough to stop the TL from tripping.

Another thing you could do, but would of course cost money would be to 
split the array up for
more inverters.

This app says that 1400 nano-Farads is an approximate point for 
problems.   That's for 50 Hz.
It will be around  6/5 worse at 60 Hz.

If you have a capacitance meter (impedance bridge or meter), you could 
measure the capacitance
of the array.  Disconnect the + and - from the inverter first of 
course.   Then, tie the plus and
minus of the combined array together and connect that to one terminal of 
the capacitance meter.
Connect the other lead of the capacitance meter to the roof and/or 
ground and it ~should~ show
the capacitance you are interested in.

This is one of those newer compromises associated with the higher 
efficiency TL inverters (anybody's,
not just SMA's of course), and also the newer code requiring TL 
(non-isolated) inverters above 300 volts
and everything else new we have to all learn about.


   Welcome to the future !      ( from Firesign Theater's   "I think 
we're all [solar] bozos on this bus" )


boB




On 3/13/2014 7:31 PM, Marco Mangelsdorf wrote:
>
> Friends,
>
> We have run across an odd technical problem at our commercial install 
> at a laundromat on our island.   Attached is the description of the 
> issue from SMA.  We don't know much except that we are switch out the 
> TL inverters to the old style US inverters because SMA has no fix for 
> the phantom ground tripping.  The issue is beyond our feeble 
> understanding.
>
> Any observations to share?
>
> Thanks,
>
> marco
>
>

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